    Patent document 1: JP-H5-016722 A
In recent years, there are proposed many systems for urging drivers to the safe driving. Such a system evaluates the degree of safe driving of the driver based on various operations of the driver and notifies the driver of the evaluation result or score. For instance, one of items evaluated by the system is as to whether or not the head light is turned on before the sunset.
With respect to the head light turning on/off, Patent document 1 describes a technology, which detects illumination around the vehicle using a sensor and the head light is automatically turned on when the illumination is less than a predetermined threshold value. The predetermined threshold value in such automatic lighting-on technology is typically set to a value such that the head light is turned on appropriately in an averaged environment; thus, for example, it is not set such that the head light is turned on when the illumination around the vehicle is still enough for driving.
In this regard, however, it is useful for safe driving to turn on the head light even though the illumination around the vehicle is still enough. Assume the case that the driver turns on the head light a little earlier than the sunset based on the own intention, without depending on the above mentioned automatic lighting-on technology. Such a case or event can be evaluated as an action which contributes to the safe driving.
In order to evaluate the lighting-on of the head light at the time prior to the sunset, the corresponding system needs to understand the sunset time accurately. For instance, one method for understanding the sunset time obtains the time when the sun sets under the horizon based on the information on the latitude and longitude of the present position of the vehicle. The calculation required for the method contains the floating point arithmetic or trigonometric functions operation to thereby relatively increase the arithmetic load. If the sunset time is continuously calculated while considering the travel movement of the vehicle, a possibility may arise which affects execution of other functions of the same system.
In addition, the change of the sunset time accompanying the travel movement of the vehicle is very small if it is considered that the moving speed of the vehicle is about 100 km/h. Therefore, when considering the utilization efficiency of the operation resource of the system, it is not appropriate to continue calculating the sunset time.